The present invention relates to a variable assist power steering apparatus and more particularly to a control valve for a variable assist power steering apparatus.
JP 53-93536 A discloses a power steering apparatus including a hydraulic operated power cylinder with a left turn and a right turn pressure chamber. A bypass line has one end connected to one of the pressure chambers and an opposite end connected to the other pressure chamber. The flow of hydraulic fluid passing through the bypass line is controlled by an externally controlled variable flow restrictor such that the rate of the flow passing through the bypass line is increased at high vehicle speed and the rate of the flow therethrough is decreased or zero at low or zero vehicle speed. In this manner, the power assist is varied in accordance with vehicle speed.
JP 56-38430 B discloses another power steering apparatus including a bypass line of the above kind.
A problem encountered with these conventional power steering apparatuses is that hydraulic fluid passing through the bypass line flows in one direction past the externally controlled variable flow restrictor during manipulation of a steering wheel counterclockwise, while it flows in the opposite direction past the externally controlled variable flow restrictor during manipulation of the steering wheel clockwise. Thus, if the externally controlled variable flow restrictor provides different characteristic on the flow hydraulic fluid passing therethrough in accordance with the direction of flow of hydraulic fluid passing through the bypass line, different steering characteristics are provided in accordance with the directions in which the steering wheel is manipulated.